In the formation of color paper it is known that the base paper has applied thereto a layer of polymer, typically polyethylene. This layer serves to provide waterproofing to the paper, as well as providing a smooth surface on which the photosensitive layers are formed. The formation of a suitably smooth surface is difficult requiring great care and expense to ensure proper laydown and cooling of the polyethylene layers. It would be desirable if a more reliable and improved surface could be formed at less expense.
In photographic papers the polyethylene layer also serves as a carrier layer for titanium dioxide and other whitener materials as well as tint materials. It would be desirable if the colorant materials rather than being dispersed throughout the polyethylene layer could be concentrated nearer the surface of the layer where they would be more effective photographically.
Prior art photographic materials have suggested coextruded layer coatings on paper base that are thicker and/or more concentrated with titanium dioxide (TiO.sub.2) and colorants than monolayers. Other high refractive index materials like zinc oxide or other finely divided solids are also used. In general, these improvements are costly and processing and coating these concentrated layers create manufacturing problems with specks, lines and surface disruptions. The highly loaded layers deteriorate the strength property of the coatings and may result in poor adhesion to the base paper or to the image bearing emulsion layer. Also, the coating speed of these layers may be lower.
The details of an invention and a description of the problems encountered with highly loaded coextruded layers is recorded in U.S. Pat. No. 5,466,519.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene in receiver sheets for thermal dye transfer. As will be shown, these materials appear to have very unique abilities to optimize thin layers for improved colorimetric performance.
An example of coextruded thin layer technology improvements and limitations is explained in U.S. Pat. No. 5,476,708 where it is proposed that sharpness improvements in photographic systems can be achieved by an untinted, unpigmented thin skin made to be used under a light sensitive emulsion. A crude correlation is made suggesting that, if the limits of coextrusion technology are pushed to the maximum, a clear layer of thickness as low as 1.5 micrometers is the optimum for optical photographic response.